Contribution of Pax1 to pathogenesis of adolescent idiopathic scoliosis

Abstract

Adolescent idiopathic scoliosis (AIS) causes adolescent-onset curvature of the spine in the coronal plane. It affects females more severely than males, and no structural deformities or causes have been attributed to the disease. AIS is a multifactorial disease, and interactions between genetic and environmental factors determine curve progression. Recently, single nucleotide polymorphisms in a putative enhancer region of PAX1 was associated with female AIS patients (Sharma et al., 2011). These SNPs have been shown to decrease gene expression by less than 50%.

Pax1 is a transcription factor that is involved in embryonic spine and intervertebral disc development. The intervertebral disc (IVD) is a fibrocartilaginous tissue that connects vertebral bones, and is composed of three compartments - the annulus fibrosus (AF), nucleus pulposus (NP) and the cartilaginous endplates (CEP) - that function together to withstand mechanical stresses such as torsion, compression and extension from movement and spinal curvature. The cells and extracellular matrix of the compartments maintain structural properties that enable mechanical loads to be evenly distributed in the IVD. However, micro-structural defects or altered cellular processes can disrupt homeostasis inside the IVD and compromise its ability to withstand mechanical stresses. Spinal curves inflict asymmetrical stresses on the disc, such that one side is compressed and the other is less compressed or more extended. Prolonged asymmetrical loading in turn also changes the biochemistry of the IVD. However, despite its importance in maintaining spinal posture, intervertebral discs have not been extensively studied as a risk factor for the initiation or progression of AIS curves. The aim of this study was to determine if partial decrease in Pax1 expression was sufficient to cause spinal curvatures or to increase the severity of curvatures.

The histology of IVDs in mice with heterozygous knockout of Pax1 revealed that partial loss of Pax1 caused mild scoliotic curvature in the thoracic region. The appearance of AF cells in the lumbar region were also affected, which suggests that spinal curves may also further progress due to decreased IVD function. In addition, when acute spinal curves were administered on Pax1-/+ mouse discs by tail looping, the extended area of the AF exhibited increased apoptosis compared to wild-type mice, which further demonstrates the inability of Pax1-/+ IVD to withstand asymmetrical loading. Finally, to elucidate how Pax1 contributes to knockout phenotypes, a list of the potential direct targets of Pax1 was generated by data mining of mouse AF and human NP transcriptomes. Candidate genes include Sox6, vinculin and talin 2, and Pax1 is possibly involved in chondrogenic differentiation of cells in the human NP. In summary, this study demonstrates that PAX1 is a potential risk factor for development and progression of scoliotic curves in female AIS patients.